1. Field of the Invention
The present invention relates to a bonding method and an apparatus therefor by which respective objects to be bonded are joined together by performing pressurization and heating with respect to a plurality of bonding objects that are in mutual contact.
2. Description of the Related Art
As one method for joining together objects to be bonded, diffusion bonding is known. More specifically, in such a method, by carrying out electrical energization or high frequency heating or the like, while pressure is imparted to bonding objects that are in mutual contact with each other, the objects are heated whereupon atoms existing at the contact region are dispersed, and as a result, both of the bonding objects are joined (bonded) together. In the case where electrical energization is performed, as the objects to be bonded (conductive bodies), all types of metallic materials, starting with steel materials, nickel alloys and copper alloys, have been selected.
In the case that diffusion bonding is carried out on the above-mentioned metallic bonding objects, when oxygen is included excessively within the bonding environment, an oxide film is formed on the surface of the metallic bonding objects, the temperature of which has been raised along with supplying electricity thereto. In this case, the diffusion of atoms is obstructed by the oxide film, and owing thereto, a concern exists in that the bonding strength (joint strength) of the bonded region cannot be assured.
In order to avert such a concern, the electrical energization and pressurizing of such metallic bonding materials generally is performed inside of a high vacuum bonding vessel, or in a bonding vessel containing an inert gas atmosphere. In the case that a high vacuum is formed in the bonding vessel, a rotary pump and a diffusing pump are both combined for carrying out evacuation of the bonding vessel, whereby setting of the pressure inside the bonding vessel on the order of 10−3 Pa is extensively carried out. In the case that further lowering of the pressure is necessary, apart from the aforementioned two pump types, a turbomolecular pump may also be combined therewith.
Further, in the case that an inert gas atmosphere is provided inside the bonding vessel, argon gas, helium gas or the like have been adopted principally as the inert gas, although depending on the circumstances, nitrogen gas has also been employed. (See, for example, Japanese Laid-Open Patent Publication No. 2006-315040.)
In the event that a diffusing pump or a turbomolecular pump is provided for establishing a high vacuum in the bonding vessel, because the cost for such pumps is high, the investment in equipment costs escalates. Further, in the case of using an inert gas atmosphere, completely exhausting the initial atmosphere (oxygen) contained initially within the bonding vessel is not easily accomplished, and thus, a defect in which generation of an oxide film cannot easily be avoided is manifested. In addition, because argon gas, helium gas and the like are comparatively high in cost, processing costs for carrying out diffusion bonding have tended to be high as well.
Furthermore, recently, attention has focused on nitrogen doped stainless steel containing a specified quantity of nitrogen to improve corrosion resistance. However, when melt welding is carried out with respect to this material, it has been reported that the nitrogen content amount at the bonding region is lowered. Of course, as a cause thereof, there is a concern that corrosion resistance also would be lowered.